The conventional manner of assembling filter tipped cigarettes involves wrapping and sticking a paper sleeve, which usually simulates cork, around a filter plug which abuts the end of a cigarette rod length so that the sleeve surrounds the plug and overlaps the cigarette rod length. Usually a filter plug of double unit length is interposed between two cigarette rod lengths and a sleeve of double length is wrapped around the double length filter plug overlapping the ends of both cigarette rod lengths. The resulting pair of cigarettes are subsequently separated by cutting the filter plug in half by means of a transverse guillotine cut.
Traditionally, defective cigarettes, for example cigarettes in which the filter plug assembly has perhaps resulted in a wrinkled sleeve or a so-called "flag" formed by a corner of a sleeve which has not been properly stuck down, have been recognised and removed by an operator watching the output of the machine. It is desirable that the monitoring should be carried out automatically, particularly with modern high speed cigarette making machines of which the output is so great that a single operator may not be able to monitor the output effectively. Attempts have been made to detect assembly faults automatically by measuring the pressure drop through the tobacco wrapper, but this is not entirely satisfactory, particularly with certain grades of cigarette paper.